To cope with the problems caused by global warming, reducing the amount of carbon dioxide is desperately needed in recent days. The automobile industry focuses on introduction of an electric car or a hybrid car to reduce the carbon dioxide emission. In this connection, an electrical device like a battery for driving a motor, which is a key element for commercialization of such cars, is actively developed.
As the battery for driving a motor, a lithium ion battery having relatively high theoretical energy has been getting an attention and has been actively developed in these days. The lithium ion battery generally has a constitution that a positive electrode in which a positive active material or the like is coated on both surfaces of a positive electrode collector by using a binder, and a negative electrode in which a negative active material or the like is coated on both surfaces of a negative electrode collector by using a binder, are connected with intervened by an electrolyte layer, and they are stored in a battery case.
For widely commercializing electric cars having the lithium ion battery, it is necessary to develop the lithium ion battery with high performance. For an electric car, in particular, it is necessary that the driving mileage by a single charge should be close to the driving mileage per gasoline fill-up of a gasoline engine car, and therefore a battery with higher energy density is required. To increase the energy density of a battery, it is necessary to increase electric capacity per unit mass of the positive electrode and the negative electrode.
As the positive electrode material which may be useful for satisfying the requirements described above, a lithium manganese-based composite oxide having a layer structure was suggested. In particular, a solid solution of electrochemically inactive Li2MnO3 having a layer structure and electrochemically active LiMO2 (M represents a transition metal including Co, Mn, and Ni or the like) having a layer structure is expected to be a candidate for the positive electrode material having high capacity which can exhibit high electric capacity of at least 200 mAh/g. In Patent Document 1, for example, xLi[Mn1/2N1/2]O2.yLiCoO2.zLi[Li1/3Mn2/3]O2 (x+y+z=1, 0<x<1, 0≦y<0.5, and 0<z<1) is disclosed as a pseudoternary solid solution.